The present invention relates to an electrophotographic material for color proofing, in particular an improvement in a photoconductive layer of the electrophotographic material for color proofing (hereinafter, sometimes referred to as a light-sensitive material).
PS plates have dominantly been used for multi color printing because of their excellence in quality, printing durability, dimensional stability and resistance to contamination. PS plates are formed by applying a photosensitive layer containing a diazo resin or o-quinone diazide as a main component onto an aluminum substrate whose surface has been treated to improve hydrophilic property thereof. The PS plates are exposed to UV rays through a color-separated and half tone film, developed to remove the photosensitive layer in non-image areas to reveal the surface of the hydrophilic aluminium plate, whereby, printing plates are obtained. A multicolor image can be obtained by mounting each printing plate prepared by the above process for each color on a printing machine and printing using an ink corresponding to each color on a sheet of same printing paper.
After it is comfirmed that a series of processes of editing, plate making and printing such as input of characters, layout, color designation, etc., are conducted as designated, and that there is no difference between color tone reproduction, etc. obtained and one designated by an orderer, necessary corrections are made, and then regular printing is conducted. This step is called press correction. The former is called "first proof", and it is conducted for the purpose of checking each process in a printing company as mentioned above. The latter is called "second proof", and it is conducted to obtain an acceptance by the orderer. The second proof is conducted in almost the same manner as one for regular printing, including the kind of inks and printing paper to be used. The resultant proof has almost the same quality as one of printed matter obtained in regular printing. Proof printing using regular printing paper to be used in regular printing is called "regular correction".
Color designation and tone reproduction of color photograph should be confirmed and corrected for each color. Especially, the proof for the orderer is generally conducted several times repeatedly. Heretofore, proof printing has been conducted using a color separated and half tone film to be used in regular printing, and a PS plate which is relatively low in cost and printing durability, for example, with the proof press, whose mechanism is similar to a press machine, of the name of "the flat bed four color proof press" made by DAINIPPON SCREEN Co., Ltd. The method mentioned above is called "press proof", which is not convenient, since it takes much time to prepare, and requires skill for an operation, and is difficult to keep quality constant.
There have been proposed some methods for color proofing which do not use the proof press having mechanism similar to one of a press machine, and therefore is easier to conduct and stable in printing. They are preferably used because of the advantages such as easier operation and stability after being used repeatedly. It is called "off press proof", and typical manners and systems thereof are disclosed in Bulletin of the Printing Society of Japan 24, No. 3, page 32 (1989). Examples of the systems include systems available in the trade names of "COLOR ART" by Fuji Photo Film Co., Ltd., "CROMALIN" by Dupont, and "MATCHPRINT" by 3M. The systems use a color separated film, but do not use the proof press mentioned above.
The color separated film is generally prepared using a color scanner. The scanner reads data from original, and conducts color separation, formation of half tone, and tone reproduction control to output digital data. A silver halide film is scan-exposed with laser beams modulated by the digital data, developed, fixed and dried to obtain a positive or negative color separated film for multi color printing.
Recently, an electronic plate making system called "total color scanner" has been developed and been utilized. The system conducts procedures up to retouching or plate collecting therein and therefore the system requires first proof capable of examining a resultant quality, more rapidly than that heretofore in use. If resultant qualities such as a manner, a layout, quality of color separation for some patterns, tone reproduction are judged with a :ress correction method heretofore in use, efficiency of the syustem will be seriously lowered, since it takes a long time for output to color separation film and the preparation of press proof or off press proof as prior art proofing.
Since an image data in the system is available as a digital information, some direct digital color proof systems which do not use proofing heretofore in use have been proposed. Details thereof are described in "Printing Information", April, page 2 et seq. (1991). Examples of such systems include a silver halide system, an electrostatic toner system (a wet electrophotographic system), an ink-jet system and a thermal sublimation transfer system.
Among them, the wet electrophotographic system is especially excellent in precision, image quality and tone reprodution and advantageous for the reason that any type of paper can be used for the system so that regular paper correction can be conducted, since, in the system, a photoconductive element is charged, exposed to laser beam and developed with every color toner comprising a pigment having four colors, i.e. cyan, magenta, yellow and black dispersed in an electrical insulating liquid, and then, the resulting toner image is transferred.
However, it is quite difficult to transfer a toner image from the surface of the photosensitive plate directly to regular paper. Japanese Patent laid open application (hereinafter reffered to as "J. P. KOKAI") No. Hei 2-272469 discloses a technique which have solved such problems, but includes several steps.
There have been proposed to provide a releasable transfer layer on the surface of the photosensitive element, to form a toner image on the transfer layer and to transfer the image together with the transfer layer to regular paper in J. P. KOKAI Nos. Sho 61-174557 (Japanese Patent publication Hei 2-43185), Hei 1-112264, Hei 1-281464, and Hei 3-11347. Among them, the system disclosed in J. P. KOKAI No. Sho 61-174557 is only practically used. However, in the system, photoconductive layer is exposed to light through a transparent substrate (a polyethylene terephthalate film), and therefore, the system requires that the photoconductive layer is also transparent. In addition, it is not reusable, Therefore, the system is expensive.
J. P. KOKAI Nos. Hei 1-112264 and Hei 1-281464 disclose an image forming method or a color printing method which uses a photosensitive paper comprising a photosensitive element on which a releasable layer and an adhesive layer are applied in order, and which is characterized in that a toner image is developed and transferred to regular paper with the releasable layer.
However, it is difficult to form such a photosensitive layer uniformly with a practical size and it is also difficult to transfer a toner image without damage.
The same will apply to the method disclosed in J. P. KOKAI No. Hei 3-11347 which uses a recording medium wherein a releasable overcoating layer is applied onto a photoconductive layer.
In addition to the problem of reduced quality of the image arising in the transfer process as explained above, the problem of reduced quality of the image arising in the electrophotographic imaging process is desired to be solved. The problem is considered to be due to deterioration of electrostatic properties of the electrophotographic material.
We have studied properties of electrophotographic materials comprising a known photoconductive layer under various environmental conditions for imaging (e.g. higher or lower temperature, or higher or lower humidity), and have found that electrophotographic properties of the materials (especially dark charge retention property and photosensitivity) were lowered, and good image qualities could not be provided stably in some cases.
When electrophotographic materials for color proofing are exposed With semiconductor laser beams by a scanning exposure method in a direct digital system, they are required to have higher electrostatic properties, especially dark charge retention property and photosensitivity, since longer exposure time is necessary and exposure intensity is limited in such a process in compared with a conventional overall simultaneous exposure system using visible lights.
However, the electrophotographic properties of the known electrophotographic materials mentioned above are deteriorated from variation of environmental conditions. Accordingly, when they are exposed with semiconductor laser beams by a scanning exposure method, the image qualities of the resultant color proofs will be deteriorated, for example, background contamination, disappearance of fine lines and batter of characters will occur in some cases.